A plastic cap comprising a top plate and a skirt hanging down from the peripheral edge of the top plate, which are integrally formed of a plastic material, and a clamping mechanism attached to the inner circumferential face of the skirt, is widely used as the cap for various bottled products, because sealing is accomplished between a vessel mouth and the cap without using a liner or packing. However, an ordinary plastic cap is still insufficient in the pressure-resistant sealing property for a content having an autogeneous pressure, such as a carbonated drink. A plastic cap excellent in the pressure-resistant sealing property, which can be applied to a carbonated drink vessel, has already been proposed. For example, Japanese Patent Application Laid-Open Specification No. 73551/83, Japanese Patent Application Laid-Open Specification No. 216552/83 and Japanese Patent Application Laid-Open Specification No. 187552/84 disclose a plastic cap having a seal lip or flap piece of a specific shape attached to the inner face of a top plate at a portion to be engaged with a vessel mouth.
In each of known caps of this type having a pressure-resistant sealing property, the above-mentioned seal lip or flap piece is engaged with the vessel mouth over a considerable distance in the direction of the vessel mouth, and as the vessel mouth is inserted, the engaging pressure between the vessel mouth and the seal lip or flap piece increases.
However, in the plastic cap of the conventional engagement system, deformation of the seal lip or flap piece is large and the engagement is produced over a considerable distance in the direction of insertion of the vessel mouth. Therefore, plastic deformation of the sealed portion is often caused, and the high initial sealing pressure is gradually reduced and the cap is defective in that a high durable sealing pressure as expected cannot be obtained.
In the case where the plastic cap of the conventional engagement system is applied to a carbonated drink-filled vessel, troubles such as blow-off of the content and spring-out of the cap are often observed. Namely, it is known that in the case where a canned or bottled product filled with a carbonated drink or beer is opened, if the gas in the head space is gradually released, blow-off of the content is controlled, but if opening is performed abruptly in a moment, blow-off of the content is caused. In the cap of the above-mentioned engagement system, since the seal lip or flap piece is engaged with the vessel mouth over a certain distance, even if the cap is turned for opening, sealing is not released and the gas in the head space is not released. It is considered that for this reason, troubles such as blow-off of the content and spring-out of the cap are caused.